Mycobacteria of Mycobacterium scrofulaceum type isolated from rat or mice lepromata are not the aetiologic agents of murine leprosy.

Mycobacterial strains M.M4 and M.EY3 were isolated from a mouse leproma, respectively, on Kl-1 and Ogawa egg-yolk medium. Strain M.M4 was scotochromogenic and produced a yellow pigment. Young cultures were non-acid-fast and became acid-fast during the exponential growth phase. Primary cultures of strain M.M4 did not grow on conventional culture media. However, the subcultures grown in Kl-1 medium were easily subcultured in the homologous media as well as on Lowenstein, Sauton and Dubos media. Cultures M.M4 were insensitive to isoniazid and p-nitrobenzoic acid. Both scotochromogenic strains M.R56 and M.M4 did not produce murine leprosy disease in susceptible animals like BALB/C and C3H mice. Primary as well as the subcultures of strain M.EY3 grew only on the Ogawa egg-yolk medium. Colonies of this strain were creamy white and unlike other mycobacteria, produced rust-like brown spots on the medium at the sites of colonies. In vitro grown cultures M.EY3 were sensitive to isoniazid and p-nitrobenzoic acid. BALB/C and C3H mice infected with M.EY3 cultures developed lepromata similar to those produced by the in vivo grown M. lepraemurium. Both M.R56 and M.M4 scotochromogenic cultures grown in liquid Kl-1 medium are not the causative agents of murine leprosy. On the other hand, cultures M.EY3 grown on Ogawa egg-yolk medium possess characteristics similar to M.lepraemurium and are, in fact, the aetiologic agents of murine leprosy.